Inflammatory Regulation of Voltage-dependent Sodium Channels in Dental Pulp Cells

Objectives: Transmembrane ionic signaling regulates many cellular processes in both physiological and pathological settings. Few findings concerning ionic signaling in dental pulp cells, however, have been obtained. Therefore, we investigated the biophysical properties of time- and voltage-dependent Na+ channels in immortalized human dental pulp (HDP) cells. Methods: Na+ channel activities were recorded by whole-cell patch-clamp recordings. We also investigated mRNA expression of voltage-dependent Na+ channel α-subunit Nav1.x and the effect of bradykinin (BK) on intracellular Ca2+ signaling. Results: Patch-clamp recordings revealed that the mean resting membrane potential (Erest) in the HDP cells was -29 mV. Depolarizing steps from a holding potential of -70 mV activated transient voltage-dependent inward currents with rapid activation/inactivation properties. No current was recorded at a holding potential of -40 mV. Fast activation kinetics exhibited strong dependence on membrane potential, whereas fast inactivation kinetics did not. Steady-state inactivation was described by a Boltzmann function with a half-maximal inactivation potential of -60 mV, indicating that, while the channels were completely inactivated at physiological Erest, they could be activated when the cells were hyperpolarized. Inward currents were sensitive to tetrodotoxin. HDP cells expressed Nav1.1 to 1.9, and BK dose-dependently up-regulated expression of Nav1.1, 1.2 and 1.8. BK also activated intracellular Ca2+-releasing pathways. Conclusion: These results indicate that HDP cells express voltage-dependent Na+ channels which are up-regulated by BK. During inflammation, chemical mediators mobilize intracellular Ca2+. This activates K+ channels, driving cellular hyperpolarization. Hyperpolarized membrane potentials prepare Na+ channels for activation. Thus, Na+ channels play an important role in driving cellular functions in HDP cells by increasing expression and channel opening during dental pulp inflammation.